- See also: Electro-optical tracking
Electro-optical MASINT is a subdiscipline of measurement and signature intelligence (MASINT), which has similarities to but complements imagery intelligence (IMINT). The basic model of IMINT is taking a photograph, perhaps using techniques that record information not visible to the unassisted human eye, but a picture nonetheless. It may take significant skill to interpret objects in that picture, but the interpretation still depends on visual metaphors such as shape, size, and shadow.
Electro-optical MASINT helps validate the elements of that picture, so that, for example, the analyst can tell if an area of green is vegetation or camouflage paint. Electro-optical MASINT also generates information on phenomena that emit, absorb, or reflect electromagnetic energy in the infrared, visible light, or ultraviolet spectra, phenomena where a "picture" is less important than the amount or type of energy reported. For example, a class of satellites, originally intended to give early warning of rocket launches based on the heat of their exhaust, reports energy wavelengths and strength as a function of location(s). There would be no value, in this specific context, to seeing a photograph of the flames coming out of the rocket.
Subsequently, when the geometry between the rocket exhaust and the sensor permits a clear view of the exhaust, IMINT would give a visual or infrared picture of its shape, while electro-optical MASINT would give, either as a list of coordinates with characteristics, or a "false-color" image, the temperature distribution, and spectrometric information on its composition. In other words, MASINT may give warning before characteristics visible to IMINT are clear, or it may help validate or understand the pictures taken by IMINT.
MASINT techniques are not limited to the United States, but the U.S. distinguishes MASINT sensors from others more than do other nations. According to the United States Department of Defense, MASINT is technically derived intelligence (excluding traditional imagery IMINT and signals intelligence SIGINT) that – when collected, processed, and analyzed by dedicated MASINT systems – results in intelligence that detects, tracks, identifies, or describes the signatures (distinctive characteristics) of fixed or dynamic target sources. MASINT was recognized as a formal intelligence discipline in 1986. Another way to describe MASINT is "a "non-literal" discipline. It feeds on a target's unintended emissive byproducts, the "trails" of thermal energy, chemical or radio frequency emission that an object leaves in its wake. These trails form distinct signatures, which can be exploited as reliable discriminators to characterize specific events or disclose hidden targets"
As with many branches of MASINT, specific techniques may overlap with the six major conceptual disciplines of MASINT defined by the Center for MASINT Studies and Research, which divides MASINT into Electro-optical, Nuclear, Geophysical, Radar, Materials, and Radiofrequency disciplines.
MASINT collection technologies in this area use radar, lasers, staring arrays in the infrared and visual, to point sensors at the information of interest. As opposed to IMINT, MASINT electro-optical sensors do not create pictures. Instead, they would indicate the coordinates, intensity, and spectral characteristics of a light source, such as a rocket engine, or a missile reentry vehicle. Electro-optical MASINT involves obtaining information from emitted or reflected energy, across the wavelengths of infrared, visible, and ultraviolet light. Electro-optical techniques include measurement of the radiant intensities, dynamic motion, and the materials composition of a target. These measurements put the target in spectral and spatial contexts. Sensors used in electro-optical MASINT include radiometers, spectrometers, non-literal imaging systems, lasers, or laser radar (LIDAR).
Observation of foreign missile tests, for example, make extensive use of MASINT along with other disciplines. For example, electro-optical and radar tracking establish trajectory, speed, and other flight characteristics that can be used to validate the TELINT telemetry intelligence being received by SIGINT sensors. Electro-optical sensors, which guide radars, operate on aircraft, ground stations, and ships.
The major subdisciplines of electro-optical MASINT are:
Airborne Electro-Optical Missile Tracking MASINT
U.S. RC-135 COBRA BALL aircraft have MASINT sensors that are "...two linked electro-optical sensors -- the Real Time Optics System (RTOS) and the Large Aperture Tracker System (LATS). RTOS consists of an array of staring sensors encompassing a wide field of regard for target acquisition. LATS serves as an adjunct tracker. Due to its large aperture, it has significantly greater sensitivity and resolving power than the RTOS, but is otherwise similar.
There is a broader program to standardize the architecture of the various RC-135 aircraft, so that there will be greater commonality of parts, and some ability to switch missions: a COBRA BALL will be able to carry out some SIGINT missions of the RIVET JOINT RC-135.
COBRA BALL cues the COBRA DANE ground radar and the COBRA JUDY ship-based radar. See Radar MASINT. This application is for technical intelligence; see Long-range missile tracking for the use of these sensors in ballistic missile defense.
Electro-optical artillery detection MASINT
Both electro-optical and radar sensors have been coupled with acoustic sensors in modern counter-artillery systems. Electro-optical sensors are directional and precise, so need to be cued by acoustic or other omnidirectional sensors. The original Canadian sensors, in the First World War, used visual observation and plotting of flash, as well as acoustic sensors.
Complementing counter-mortar radar is the Israeli Purple Hawk mast-mounted electro-optical sensor, which detects mortars and provides perimeter security. The device, remotely operated via fiber optics or microwave, is intended to have a laser designator.
Rocket Launch Spotter
A newer U.S. system couples an electro-optical and an acoustic system to produce the Rocket Artillery Launch Spotter (RLS). RLS combines components from two existing systems, the Tactical Aircraft Directed Infra-Red Countermeasures (TADIRCM) and the UTAMS . The two-color infrared sensors were originally designed to detect surface-to-air missiles for TADIRCM. Other TADIRCM components also have been adapted to RLS, including the computer processors, inertial navigation units (INU), and detection and tracking algorithms.
It is an excellent example of automatic cueing of one sensor by another. Depending on the application, the sensitive but less selective sensor is either acoustic or nonimaging electro-optical. The selective sensor is forward-looking infrared (FLIR).
RLS uses two TADIRCM sensors, an INU, and a smaller field-of-view single-color (FLIR) camera on each tower. The INU, which contains a GPS receiver, allows the electro-optical sensors to align to the azimuth and elevation of any detected threat signature.
The basic system mode is for rocket detection, since a rocket launch gives a bright flare. In basic operation, RLS has electro-optical systems on three towers, separated by 2 to 3 kilometers, to give omnidirectional coverage. The tower equipment connects to the control stations using a wireless network.
When a sensor measures a potential threat, the control station determines if it correlates with another measurement to give a threat signature. When a threat is recognized, RLS triangulates the optical signal and presents the Point of Origin (POO) on a map display. The nearest tower FLIR camera then is cued to the threat signature, giving the operator real-time video within 2 seconds of detection. When not in RLS mode, the FLIR cameras are available to the operator as surveillance cameras.
Mortar launches do not produce as strong an electro-optical signature as does a rocket, so RLS relies on acoustic signature cueing from an Unattended Transient Acoustic Measurement and Signal Intelligence System (UTAMS). There is an UTAMS array at the top of each of the three RLS towers. The tower heads can be rotated remotely.
Each array consists of four microphones and processing equipment. Analyzing the time delays between an acoustic wavefront’s interaction with each microphone in the array UTAMS provides an azimuth of origin. The azimuth from each tower is reported to the UTAMS processor at the control station, and a POO is triangulated and displayed. The UTAMS subsystem can also detect and locate the point of impact (POI), but, due to the difference between the speeds of sound and light, it may take UTAMS as long as 30 seconds to determine the POO for a rocket launch 13 km away. This means UTAMS may detect a rocket POI prior to the POO, providing very little if any warning time. but the electro-optical component of RLS will detect the rocket POO earlier.
Short-range aircraft and missile tracking
Infrared tracking has long been used for missile guidance, first with air-to-air missiles such as the AIM-9 Sidewinder, but also with MANPADS such as the 9K31 Strela-1 (Western: SA-9 GASKIN). Pure infrared sensors, however, are vulnerable to being decoyed by hot flares.
While newer infrared seekers use additional wavelengths and pattern recognition that can discriminate flares, another approach has been to add ultraviolet sensors, or to replace them completely. For example, the FIM-92 Stinger short-range antiaircraft missile uses "dual-band" infrared and ultraviolet guidance. The AN/AAR-54, a representative missile warning receiver, primarily intended for use on aircraft, uses pure ultraviolet detection, although missile warning receivers increasingly use multiple electro-optical wavelengths, and may cue radar onto the missile track.
Long-range missile tracking
The US, in 1970, launched the first of a series of space-based staring array sensors that detected and located infrared heat signatures, typically from rocket motors but also from other intense heat sources. Such signatures, which are associated with measurement of energy and location, are not pictures in the IMINT sense. Currently called the Satellite Early Warning System (SEWS), the program is the descendant of several generations of Defense Support Program (DSP) spacecraft, which is operated by the Fourteenth Air Force. Originally, DSP was known by the classified name Program 949, and, after that became known, Program 647.
Originally intended to detect the intense heat of an ICBM launch, this system proved useful at a theater level in 1990-1991. It detected the launch of Iraqi SS-1 SCUD missiles in time to give early warning to potential targets.
First used for technical intelligence, the RC-135 COBRA BALL system now has the potential to significantly assist theater-based ballistic missile defense, by giving detailed tracking after launch. Similar sensors may be mounted on future unmanned aerial vehicles.
Next-generation satellite platforms
- See also: Space-Based Infrared System
DSP is to be succeeded by the Space-Based Infrared System (SBIRS). DSP and SBIRS give worldwide coverage, but are really optimized for ICBMs. SBIRS High continues to be the name for the part of this system that operates from geosynchronous orbit, but the names of the low earth orbit components keep changing.
A lower-altitude constellation can give faster warning of short-range ballistic missile launches, important at the theater level. The U.S. system for this purpose has gone through several redirections and name changes, including SBIRS-Low, BRILLIANT EYES, and now Space Tracking and Surveillance System (STSS).  As opposed to DSP and SBIRS, which basically detect the point of origin (POO) of a launch and its general direction, STSS actually can track the missiles.
It has been confusing to some that while SBIRS is, indeed, an intellicence system, it is also a fundamental part of modern ballistic missile defense, its funding is not included in the budget of the Missile Defense Agency. 
Optical Measurement of Nuclear Explosions
There are several distinctive characteristics, in the range of visible light, from nuclear explosions. One of these is a characteristic "dual flash" measured by a bhangmeter. This went into routine use on the advanced Vela nuclear detection satellites, first launched in 1967. The earlier Velas only detected X-rays, gamma rays, and neutrons.
The bhangmeter technique was used earlier, in 1961, aboard a modified US KC-135B aircraft monitoring the preannounced Soviet test of Tsar Bomba, the largest nuclear explosion ever detonated. The US test monitoring, which carried both broadband electromagnetic and optical sensors including a bhangmeter, was named SPEEDLIGHT.
As part of Operation BURNING LIGHT, one MASINT system photographed the nuclear clouds of French atmospheric nuclear tests to measure their density and opacity.  This operation is borderline with Nuclear MASINT.
Bhangmeters on Advanced Vela satellites detected the what is variously called the Vela Incident or South Atlantic Incident, on 22 September 1979. Different reports have claimed that it was, or was not, a nuclear test, and, if it was, probably involved South Africa and possibly Israel. France and Taiwan have also been suggested. Only one bhangmeter detected the characteristic double-flash, although US Navy hydrophones suggest a low-yield blast. Other sensors were negative or equivocal, and no definitive explanation has yet been made public.
This discipline includes both measuring the performance of lasers of interest, and using lasers as part of MASINT sensors. With respect to foreign lasers, focus of the collection is on laser detection, laser threat warning, and precise measurement of the frequencies, power levels, wave propagation, determination of power source, and other technical and operating characteristics associated with laser systems strategic and tactical weapons, range finders, and illuminators. 
In addition to passive measurements of other lasers, the MASINT system can use active lasers (LIDAR) for distance measurements, but also for destructive remote sensing that provides energized material for spectroscopy. Close-in lasers could do chemical (i.e., materials MASINT) analysis of samples vaporized by lasers.
Laser systems are largely at a proof of concept level. One promising area is a synthetic imaging system that would be able to create images through forest canopy, but the current capability is much less than existing SAR or EO systems.
A more promising approach would image through obscurations such as dust, cloud, and haze, particularly in urban environments. The laser illuminator would send a pulse, and the receiver would capture only the first photons to return, minimizing scattering and blooming.
Use of LIDAR for precision elevation and mapping is much closer, and again chiefly in urban situations.
- Interagency OPSEC Support Staff (IOSS) (May 1996). Operations Security Intelligence Threat Handbook: Section 2, Intelligence Collection Activities and Disciplines.
- Lum, Zachary (August 1998). "The measure of MASINT". Journal of Electronic Defense.
- Center for MASINT Studies and Research. Center for MASINT Studies and Research. Air Force Institute of Technology.
- US Army (May 2004). Chapter 9: Measurement and Signals Intelligence. Field Manual 2-0, Intelligence. Department of the Army.
- Pike, John, COBRA BALL
- Daniel W. Caldwell, Radar planning, preparation and employment of 3-tiered coverage: LCMR, Q-36 and Q-37
- Mabe, R.M. et al.. Rocket Artillery Launch Spotter (RLS).
- Jeffrey Richelson (1999), America's Space Sentinels: DSP Satellites and National Security, University of Kansas Press
- Interagency OPSEC Support Staff (May 1996), Operations Security Intelligence Threat Handbook, Section 3, Adversary Foreign Intelligence Operations
- Northrop Grumman, Space Tracking and Surveillance System (STSS)
- Fred Kaplan (4 February 2010)
- Sublette, Carey. Big Ivan, The Tsar Bomba (“King of Bombs”): The World's Largest Nuclear Weapon.
- History Division, Strategic Air Command, SAC Reconnaissance History, January 1968-June 1971
- Office of the Historian, Strategic Air Command, History of SAC Reconnaissance Operations, FY 1974
- Office of the Secretary of Defense, Unmanned Aircraft Systems Roadmap 2005-2030